A lithographic projection apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.
Lithography is widely recognized as one of the key steps in the manufacture of ICs and other devices and/or structures. However, as the dimensions of features made using lithography become smaller, lithography is becoming a more critical factor for enabling miniature IC or other devices and/or structures to be manufactured.
A theoretical estimate of the limits of pattern printing can be given by the Rayleigh criterion for resolution as shown in equation (1):
                    CD        =                              k            1                    *                      λ            NA                                              (        1        )            where λ is the wavelength of the radiation used, NA is the numerical aperture of the projection system used to print the pattern, k1 is a process dependent adjustment factor, also called the Rayleigh constant, and CD is the feature size (or critical dimension) of the printed feature. It follows from equation (1) that reduction of the minimum printable size of features can be obtained in three ways: by shortening the exposure wavelength λ, by increasing the numerical aperture NA or by decreasing the value of k1.
In order to shorten the exposure wavelength and, thus, reduce the minimum printable size, it has been proposed to use extreme ultraviolet (EUV) radiation. EUV radiation is electromagnetic radiation having a wavelength within the range of 5-20 nm, for example within the range of 13-14 nm. It has further been proposed that EUV radiation with a wavelength of less than 10 nm could be used, for example within the range of 5-10 nm such as 6.7 nm or 6.8 nm. Such radiation is termed extreme ultraviolet radiation or soft x-ray radiation. Possible sources include, for example, laser-produced plasma sources, discharge plasma sources, or sources based on synchrotron radiation provided by an electron storage ring.
EUV radiation may be produced using a plasma. A radiation system constructed to produce EUV radiation may include a laser for exciting a fuel to provide the plasma, and a source collector apparatus for containing the plasma.
The plasma may be created, for example, by directing a laser beam at a fuel, such as particles of a suitable material (e.g. tin), or a stream of a suitable gas or vapor, such as Xe gas or Li vapor. The resulting plasma emits radiation, e.g., EUV radiation, which is collected using a radiation collector. The radiation collector may be a normal-incidence radiation mirror, which receives the radiation and focuses the radiation into a beam. The source collector apparatus may include an enclosing structure or chamber arranged to provide a vacuum environment to support the plasma. Such a radiation system is typically termed a laser-produced plasma (LPP) source.
Alternatively, the plasma may be created, by injecting a droplet of the fuel liquid in an electric field, e.g., between an anode and a cathode. When the droplet is at a predetermined position, it is hit by a laser and evaporated. Subsequently, a discharge occurs between the anode and the cathode creating a plasma which emits EUV radiation.
Typically, the radiation system constructed to produce EUV radiation is manufactured separately from the lithographic projection apparatus. This means the radiation system and the lithographic projection apparatus need to be aligned and connected before the lithographic projection apparatus can be used.
It requires a substantial amount of space and electronics to manually align and connect the radiation system and the lithographic projection apparatus to each other. This space may not be available due to the presence of other parts of the radiation system and/or the lithographic projection apparatus.